Wood boards using wood particles or wood fibers as a raw material which are bonded by means of a synthetic resin binder, molded and then formed into a solid body, are known. Wood boards may be classified into particle boards and fiber boards, based on the form of the raw material. The term particle board refers to a board material in which particles of wood produced by the cutting or shaving of raw wood, such as logs or the like, are molded and formed into a solid body by means of a synthetic resin binder. Particles are obtained by the simple cutting or shaving of wood, and such particles are termed "chips" or "flakes", depending on the conditions under which they were produced. On the other hand, fibers having lengths from ones to tens of mm obtained by fiberizing wood are molded and formed into a solid body by means of a binder to obtain a fiber board. Fiber boards are uniform, so that the workability thereof is good. The form thereof is not limited to a flat surface, and the fiber boards can be made into a curved surface, and such boards can be produced in large amounts.
However, both particle boards and fiber boards possess a drawback: they lack a resistance to moisture, in that the thickness of the board varies as a result of the absorption of water after board formation. Various attempts have been made to impart resistance to moisture to such boards by means of the acetylation processing of the wood particle or wood fiber which forms the raw material thereof.
Rowell and Simonson, et. al., dried a lignocellulose material (wood flakes having a predetermined size and jute cloth), immersed these lignocellulose materials in liquid acetic acid anhydrate without the presence of a catalyst, or sprayed this lignocellulose with acetic acid anhydrate, and then maintained this material within a cylinder for a period of from 2 to 8 hours at a temperature of 120.degree. C., and thus conducted acetylation processing (EP-A-213252).
The same Rowell conducted acetylation processing by immersing wood chips having a thickness of approximately 1 mm and a maximum length of 25 mm, which had been dried in advance, in liquid acetic acid anhydrate, and then maintained these chips in liquid acetic acid anhydrate for a period of 4 to 48 hours at a temperature of 120.degree. C. (Nordic Pulp and Paper Research Journal No. 2/1986).
Furthermore, Rowell, et. al., conducted solvent-and-catalyst-free acetylation processing by predrying southern pine flakes (thickness 0.5 mm, length 64 mm, width random) and aspen flakes (thickness 0.6 mm, length 38 mm, width random), immersing the flakes in liquid acetic acid anhydrate, and then maintaining the flakes in liquid acetic acid anhydrate for a period of 2 hours at a temperature of 120.degree. C. (Journal of Wood Chemistry and Technology, 6(3), 427-448, 1986).
In the acetylation processing techniques discussed above, the catalyst-free acetylation processes of wood chips or wood flakes were carried out in the liquid acetylating agent. In such cases, the processing temperature can be raise up to the boiling point of liquid acetic acid anhydrate at the highest (the boiling point of acetic acid anhydrate is 139.degree. C.). Accordingly, the reaction rate cannot be raised, and a long processing period is necessary. Furthermore, another problem exists in: when a board is formed after acetylation, there is a persistent acetic acid odor because acetic acid remains in the wood chips or wood flakes.
Furthermore, A. D. Sheen conducted acetylation processing by immersing wood fibers in liquid acetic acid anhydrate and then maintaining these fibers in liquid acetic acid anhydrate for a period of 2 hours at a temperature of 120.degree. C., in order to obtain acetylated wood fiber on a commercial scale (Chemical Modification of Lignocellulosics, 7-8 Nov. 1992, Rotorua, New Zealand).
However, because liquid phase acetic acid anhydrate was employed, the reaction rate could not be increased, and it was difficult to raise the degree of acetylation in a short period of time, as stated above. Furthermore, when wood fiber was subjected to acetylation processing in liquid acetic acid anhydrate, the acetic acid anhydrate reacted with substances in wood fibers and were then converted to acetic acid, contaminants escaped from the wood fibers into the acetic acid anhydride, and thereby, the acetic acid anhydride became unfit for use after a single use in acetylation processing, so that a large amount of wasted chemicals was generated. Further, Sheen converted a large-scale plant for acetic acid anhydrate production so as not to generate wasted chemicals; however, it is not a simple matter to provide such a plant, and the provision of such a plant involves enormous costs.
The Rowell's paper (Nordic Pulp and Paper Research Journal No. 2/1986) mentioned above conducted catalyst-free acetylation processing by exposing previously dried wood chips having a thickness of approximately 1 mm and a maximum length of 25 mm to vaporized acetic acid anhydrate, and then maintaining these chips for a period of from 4 to 48 hours at a temperature of 120.degree. C.
Rowell and Youngquist, et. al., conducted catalyst-free acetylation processing by exposing wood flakes to a vapor phase acetic acid anhydrate at a temperature of 140.degree. C. for a period of 4 hours. After this 4-hour acetylation processing, the wood flakes exhibited a 15 weight percent gain (WPG). However, the degree of acetylation could not be raised even if the temperature of the acetic acid anhydrate was raised and acetylation processing was conducted for a long period of time.
Harada (Proceedings of the International Symposium on Chemical Modification of Wood, May 17-18, 1991, Kyoto, Japan) subjected a veneer (width: 75 mm, thickness: 2 mm, length: 75 mm) acetylation processing in vapor phase. In this case in which acetylation processings were conducted in two ways. in one way, it was conducted without preprocessing with a catalyst, then an acetylation degree from 14 to 17% was realized by maintaining the veneer for a period of 1 to 2 hours at a temperature of 140.degree. C. However, the reaction rate was low and furthermore, the degree of acetylation was insufficient. In the other way, Harada simultaneously disclosed an acetylation process by immersing the veneer in a catalyst (potassium acetate) and then maintaining the veneer at a temperature of 140.degree. C. for a period of from 40-90 minutes. A degree of acetylation was 20 WPG. However, because a catalyst was used, the number of processes increased. Furthermore, as catalyst are remaining in the veneer after acetylation process, an acetic acid odor was generated.
Nishino conducted acetylation processing by exposing wood particles (small specimen: 30 mm.times.26 mm.times.5 mm) to acetic acid anhydrate vapor (Journal of the Japan Wood Research Society, Volume 37, No. 4, pages 370-374, 1991). However, because the wood particles were acetylated in the vapor phase, the reaction rate was low, and a long period of time was required to obtain the desired acetylation degree.
The various vapor phase acetylation processing methods for wood particles described above exhibited results which were inferior to those exhibited by liquid phase acetylation processing.